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Investigation of dynamic properties of carbonate/calcareous soils is important in earthquake and offshore engineering as these soils are commonly encountered in large-scale projects related with energy geomechanics and land reclamation. In this study, the stiffness and stiffness anisotropy of two types of calcareous sands (CS) from the Western Australia and the Philippines were examined using bender elements configured in different directions in stress path setups. Stiffness measurements were taken on specimens subjected to constant p′ compression/extension and biaxial stress paths and additional tests were performed on three types of silica sands with different geological origins and particle shapes, which were used as benchmark materials in the study. Compared with the three brands of silica sands, the stiffness of the CS was found to be more significantly influenced by anisotropic loading; an important observation of the experimental results was that stress anisotropy had different weighted influences on the stiffness in different directions, thus influencing stiffness anisotropy. Comparisons were made between the specimens subjected to complex loading paths, and respected model parameters as suggested from published expressions in the literature. These comparisons further highlighted that calcareous soils have different responses in terms of stiffness, stiffness anisotropy and loading history, compared with that of silica-based sands.

The coefficient of restitution (COR) is an important input parameter in the numerical simulation of granular flows, as it governs the travel distance, the lateral spreading, and the design of barriers. In this study, a new custom-built micromechanical impact loading apparatus is presented along with impact experiments on engineered and natural materials. The COR and energy loss of various grains and base block combinations are studied, including fairly regular-shaped Leighton Buzzard sand (LBS) grains as a natural soil and granite or rubber as base blocks, apart from the use of engineered materials for the grains (chrome steel balls, glass balls) and blocks (stainless steel, brass). The repeatability of the new micromechanical impact loading apparatus was checked by impacting chrome steel balls on stainless steel block. In all the test combinations, the higher and lower values of the COR are found for granite block (ranging between 0.75 and 0.95) and rubber block (ranging between 0.37 and 0.44) combinations, respectively. For the tested grain–block combinations, lower values of COR were observed for impacts between materials of low values of composite Young’s modulus. However, within the narrow range of composite surface roughness of the tested grain–block interfaces no particular trend was observed in the COR values. Compared to glass balls and chrome steel balls, greater scatter in the COR values is observed for natural sand grains. This is due to the variation of the elastic and morphological characteristics among individual LBS grains.

The shearing behaviour of reproduced flat LBS grains artificially bonded with ordinary Portland cement (OPC) and plaster of Paris (PP) was examined using micromechanical experiments. Monotonic shearing tests showed a distinct variation in the load–displacement relationship at low, medium and high normal loads, and a nonlinear shear strength envelope was proposed. For OPC-bonded sand grains, a brittle–ductile transition at 20–30 N normal load was observed and three breakage mechanisms in shearing (chipping, shear cracks and crushing) were distinguished in accordance with the changes in the load–displacement curves. OPC-bonded sands showed a predominant dilation at lower normal loads, whereas PP-bonded sands were highly compressive. Based on previously published works using element-scale tests, a new mechanism for dilation under micromechanical testing was proposed in the study. Cyclic shearing tests were conducted on OPC-bonded sands, and the effects of increased displacement amplitude and normal load were highlighted.

Drained triaxial shearing tests were performed on a well-graded compressive sand (completely decomposed granite, CDG) and its mixtures with granulated rubber tires to investigate the effects of rubber size and content on their mechanical behaviour. Three sizes of rubber particles, GR1, GR2, and GR3, were used with size ratios to CDG (D50,rubber : D50,CDG) of 0.9, 3.5, and 7.2, respectively, and the rubber content ranged from 0% to 30%. The results show that for CDG–GR1 mixtures, the strength decreases with increasing rubber content, while for CDG–GR2 and CDG–GR3 mixtures, the strength decreases only at 10% rubber content and then increases markedly with increasing rubber content. The increase of strength is mainly because the inclusion of large rubber particles widens the particle size distributions of the mixtures, resulting in denser packings. The denser packings also lead to a decrease in compressibility. At larger size ratio and higher rubber content, the CDG–rubber mixtures show higher shear strength and lower compressibility than pure CDG, which indicates the CDG–rubber mixtures are very suitable to be used as filling materials.

Granular materials are of interest in various fields of engineering and science and they commonly display rough morphologies which govern their constitutive behavior. One of the emerging applications of granular materials is their use as proppants in hydraulic fracturing and the present study provides an investigation into the normal contact behavior of three types of proppant simulants using micromechanical-based experiments. As the complex processes in hydraulic fracturing necessitate accurate modeling of the order of nano-to-micrometers of displacements, a micromechanical investigation of grain contact response accounting for extremely small ranges of deformations is necessary to provide insights with potential applications in contact mechanics modeling. The proppant simulants used in the present study are composed of quartz sand grains from different origins with variations of their local surface morphologies and nano-to-micro scale roughness. Based on their local radii, which govern the contact behavior of non-conforming surfaces, the proppant simulants would approximately correspond to mesh 16/30–20/40. In view of their global radii, these grains would be considered on the upper bound of potential sizes used in hydraulic fracturing. In terms of normal contact response, the data were fitted using two contact mechanics models and a discussion is elaborated on their applicability in simulating the normal load–displacement behavior of grains at their contacts. It is shown that surface asperities play an important role in the constitutive behavior of the grain contacts and control the different zones of response as observed from both experiments and analytical expressions. Natural grains used as proppants may display smoother surfaces compared with other geological materials, such as weathered rocks or crushed rocks. However, their roughness is still an important influencing factor that should be considered in the simulation of multi-scale problems, especially when micrometer-level displacements are important to be encountered. Based on differentiation of the normal force for a given range of displacements, tangent stiffness was derived and the data showed that tangent normal stiffness (KN) increases with normal displacement. Thus, the load (or stress) state of a given contact is important to provide appropriate values of the ratio (KT/KN), where the tangential stiffness (KT) decreases rapidly with the increase of the sliding displacement.

In materials science and engineering, a significant amount of research has been carried out using indentation techniques in order to characterize the mechanical properties and microstructure of a broad range of natural and engineered materials. However, there are many unresearched or partly researched areas, such as, for example, the investigation of the shape of the indentation load–displacement curve, the associated mechanism in porous materials with clastic texture, and the influence of the texture on the constitutive behavior of the materials. In the present study, nanoindentation is employed in the analysis of the mechanical behavior of a benchmark material composed of plaster of Paris, which represents a brand of highly porous-clastic materials with a complex structure; such materials may find many applications in medicine, production industry, and energy sectors. The focus of the study is directed at the examination of the influence of the porous structure on the load–displacement response in loading and unloading phases based on nanoindentation experiments, as well as the variation with repeating the indentation in already indented locations. Events such as pop-in in the loading phase and bowing out and elbowing in the unloading phase of a given nanoindentation test are studied. Modulus, hardness, and the elastic stiffness values were additionally examined. The repeated indentation tests provided validations of various mechanisms in the loading and unloading phases of the indentation tests. The results from this study provide some fundamental insights into the interpretation of the nanoindentation behavior and the viscoelastic nature of porous-clastic materials. Some insights on the influence of indentation spacing to depth ratio were also obtained, providing scope for further studies.

In the present study, the micromechanical behaviour at the interfaces of sand grains coated with kaolinite powder is examined. Two different classes of coated grains were produced, one with lighter coating and one with heavier coating. The interface micromechanical experiments were conducted on pairs of grains and the emphasis was placed on the normal and tangential load-displacement relationships and the inter-particle friction of the different pairs highlighting the differences between uncoated and coated grains and also between the different classes of coating. All the experiments were conducted using a micromechanical custom-built loading apparatus and for the analysis of the data, analytical models from the literature were used, where adequate modifications were applied in specific in the tangential direction to match with the experimental results. Iteration processes, image analyses and microscopic image observations were adopted in the study of the frictional behaviour of the grains as well as the study of the damage of the surfaces. Work done and rate of work done concepts were introduced and linked to the damage of the surfaces of the grains and the role of repeating the shearing tests following the same shearing paths were particularly emphasized. The results from the study can provide some useful insights into the grain-scale behaviour of soil grains as well as input parameters in DEM studies.

The study reports on the small-strain dynamic behavior of two types of carbonate sands from Western Australia and the Philippines. Basic characterization of the soils was performed in terms of specific gravity, grading information, angle of shear strength at critical state, particle shape characterization and composition analysis. Piezo-element inserts were utilized to carry out the dynamic tests. For the Western Australia (WA) carbonate sand, both bender and extender element tests were performed, thus the shear modulus, the Young’s modulus and the Poisson’s ratio were examined. Both vertical and lateral bender element tests were performed on a set of specimens from the Philippines (PH) carbonate sand to study the shear modulus and, from which, no fabric anisotropy of the reconstituted specimens was found. It was observed that the overconsolidated specimens had higher stiffness than those during the first loading stages for both carbonate sands. In the pressure range of the study, grain breakage was small and its effect on the behavior of the samples was almost negligible. Empirical equations in the literature proposed from quartz sands could not predict the stiffness of the carbonate soils satisfactorily. In this regard, a preliminary study was carried out adopting the assumed void ratio that only considers the inter-particle voids instead of the summation of inter- and intra-particle voids; based on this concept, the predicted and measured stiffness (including both small-strain shear modulus, Gmax and small-strain Young’s modulus, Emax) were found to be satisfactorily close.

The study of rate and shearing effects is of major interest in geomechanics and petroleum engineering research. In this paper, a new micromechanical apparatus is presented along with calibration and reliability tests, which is designed in a way that the interface behavior of granular materials can be examined for a broad range of shearing velocities of five-orders of magnitude utilizing a dynamic data-logger and high-resolution sensors. The results showed that the shearing velocity between 0.4 and 1,340 mm/h did not influence the response of dry grain-grain interfaces including engineered and natural grains, but it significantly affected the frictional behavior of composite grain-rubber interfaces.

We investigate the inter-particle tangential force–displacement behavior of a broad range of granular materials. The major experiments are conducted using a custom-built micromechanical loading apparatus and the emphasis of the work is placed on the microslip displacement. We show that for all the materials tested, the microslip displacement increases with the increase in normal force and that extended threshold displacements are observed for rougher and softer grains. An analytical expression proposed in the literature is modified, incorporating material micro-hardness in a normalized form, to establish an expression which can be used in micromechanical-based analysis of problems involving geological materials.

Recycled rubber in granulated form is a promising geosynthetic material to be used in geotechnical/geo-environmental engineering and infrastructure projects, and it is typically mixed with natural soils/aggregates. However, the complex interactions of grains between geological materials (considered as rigid bodies) and granulated rubber (considered as soft bodies) have not been investigated systematically. These interactions are expected to have a significant influence on the bulk strength, deformation characteristics, and stiffness of binary materials. In the present study, micromechanical-based experiments are performed applying cyclic loading tests investigating the normal contact behavior of rigid–soft interfaces. Three different geological materials were used as “rigid” grains, which have different origins and surface textures. Granulated rubber was used as a “soft” grain simulant, this material has viscoelastic behavior and consists of waste automobile tires. Ten cycles of loading–unloading were applied without and with preloading (i.e., applying a greater normal load in the first cycle compared with the consecutive cycles). The data analysis showed that the composite sand–rubber interfaces had significantly reduced plastic displacements, and their behavior was more homogenized compared with that of the pure sand grain contacts. For pure sand grain contacts, their behavior was heavily dependent on the surface roughness and the presence of natural coating, leading, especially for weathered grains, to very high plastic energy fractions and significant plastic displacements. The behavior of the rigid–soft interfaces was dominated by the rubber grain, and the results showed significant differences in terms of elastic and plastic fractions of displacement and dissipated energy compared with those of rigid interfaces. Additional analysis was performed quantifying the normal contact stiffness, and the Hertz model was implemented in some of the rigid and rigid–soft interfaces.

We investigated experimentally the tribological behavior of tailing grain contacts using a micromechanical apparatus which allows high precision of force and displacement measurements to derive contact stiffness. A technique was developed to apply biopolymer coating to the grain surfaces and the emphasis of the study was placed on the investigation of the influences of saturation conditions, the presence of polymer-based coating, and the abrasion on the frictional behavior of the grains. Material characterization was based on interferometry, micro-indentation and elemental composition analyses. Elastoplastic displacements dominated the first cycles of normal loading and the Young’s modulus was interpreted based on different contact models. The tailing grains showed significantly higher inter-particle friction compared with that of quartz grains. Three major characteristics which influenced the frictional behavior of the grain contacts were the abrasion, which was more dominant in the first loading cycles, the high roughness of the grains and the presence of the biopolymer coating which increased significantly the friction.

Understanding the evolution of dynamic properties of sand subjected to induced anisotropy is indispensable for an accurate seismic analysis of geostructures. In this study, several bender element and hollow cylinder experiments along various anisotropic stress paths are performed on two types of sands to evaluate the soil dynamic properties over a wide range of small to large strain levels. Accordingly, the influences of both the major principal stress direction (α-direction) and the intermediate principal stress, b=(σ2-σ3)/(σ1-σ3), are examined. The data suggested that α-direction affects both stiffness and damping ratio, especially at higher confining pressures. However, b-parameter has little influence on stiffness and a fairly negligible impact on damping ratio. Based on the experimental results, an empirical model published in the literature is extended by introducing correction factors so as to incorporate the significant contribution of induced anisotropy into the predictions of shear stiffness and damping ratio of sands.

Despite the significant progresses in discrete-based numerical studies of granular materials, most efforts have been placed in the study of large deformation problems and there are many unresearched areas in the multi-scale analysis and the influence of grain-scale parameters on the bulk behavior of granular systems in the regime of small strains. In this work, we present a numerical investigation into the small and small-to-medium strain shear modulus of sands performing DEM analyses, exploring the influence of grain-scale parameters, and attempting to link micromechanical-based quantities with the macroscopic behavior of granular materials. For the calibration, we used real experimental grain-scale data for the contact stiffness and interparticle friction, and resonant column macroscopic test results on a quartz sand were used as the benchmark model sample. Subsequently, a parametric DEM study was carried out investigating the influence of Young’s modulus (and contact stiffness) as well as the interparticle friction on macroscopic stiffness and stiffness reduction curves, and additional insights were obtained based on the inferred contact forces and force networks, coordination number and fabric anisotropy. The numerical results contribute to understand the differences between different sand types as reported in the literature in the regime of small-to-medium strains.

Grain scale tests can provide useful information about geological materials interface properties and to understand the possible role of geological environments on their behavior. This paper presents an experimental micromechanical study investigating the behavior of two quartz sands at the grain scale, namely River sand and Beach sand which are obtained from two different geological environments. The results of the study are compared with Leighton Buzzard sand (LBS) quartz grains, with the latter material being extensively examined in previous research works. Basic characterization of the grains is conducted in terms of chemical composition, particle shape and roughness. It was observed that the grains of the Beach sand are relatively rougher and less rounded compared to the River sand grains. A custom-built new generation micromechanical apparatus was used to quantify the micromechanical behavior of the sand grains at their contacts from the two different materials. It was observed that both materials showed a similar range of Young's moduli, which are obtained from Hertzian fitting. The River sand grains showed fairly steady state behavior during shearing, which was not very obvious for the grains of the Beach sand. This different observation in steady state was attributed, partly, to grain morphology effects. The stiffness degradation of both sands followed similar trends. The average value of the inter-particle coefficient of friction for River sand and Beach sand grains are about 0.15 and 0.22, respectively, whereas for LBS grains the inter-particle coefficient of friction lies in between that of River sand and Beach sand. The high roughness and morphology of the Beach sand grains are factors which have contributed to the observed trends and the differences between the variable types of sands.

The development of the discrete element method (DEM) has provided important insights into the micro-scale behavior of geological materials with major applications in geotechnical, geological and petroleum engineering. Mechanical response of grain contacts including normal and tangential behavior are given as input quantities in various DEM applications. However, it was only achieved in recent years to properly capture the real soil grain behavior in the laboratory and provide useful input parameters to be further utilized in DEM analyses. In this paper, an experimental study was conducted investigating the normal and tangential contact behavior of soil grains composed of limestone sand (denoted as LS), which is a material of biogenic origin and it may find applications, for example, in offshore engineering. In their previous work (Senetakis et al. in Tribol Int 111:1–8, 2017), the authors reported on the dynamic inter-particle friction of this material and concluded that due to the smooth surface of the brand of grains they tested, the inter-particle friction was found lower in comparison to quartz grains of Leighton Buzzard sand (LBS). That study quantified friction at relatively large displacements of shearing, of the order of 80 μm or above, where a steady state sliding was observed. In the present work, the mechanical behavior of LS sand grains was enhanced with additional test data to further explore normal and tangential behavior. The focus of the study was to investigate the differences in inter-particle friction at different reference displacements, termed as small displacement range, where the behavior exhibited a peak, and large displacement range, where a fairly steady state sliding was occurred as well as to stress the differences in the observed responses between LS and LBS grains in terms of tribological and contact mechanics behavior. It is shown in the study that very high friction angles may be observed at displacements of about 10–30 μm for LS grain contacts when sheared under low normal forces, with a substantial drop of the mobilized friction leading to a steady state at much greater sliding displacements. This drop of friction with shearing displacement was not observed when the grains were sheared at higher normal forces. The morphological effects on friction were more evident at low normal forces for LS grains. In the normal loading tests, the LS sand grains exhibited significant plastic behavior. After the conduction of a number of normal loading–unloading cycles, the observed plastic deformations continued to increase, even though this observed trend was dominant at the first cycle. The observed apparent contact Young’s moduli of LS grains were found to be much lower than those of LBS grains. These observations should not be ignored by DEM modelers when the numerically created sample targets to represent a granular material such as crushed LS.

Recycled concrete aggregate (RCA) is a promising substitute for natural aggregates and the reuse of this demolished material would benefit the construction projects both economically and environmentally. A difficulty associated with the prediction of the behavior of RCA is because of its multi-composition which is linked to the grain size in consideration. In this study, a comprehensive laboratory testing program is conducted on different fractions of RCA for which the ratio of silicon over calcium increases for the coarser fractions due to the limitation of the cement mortar inclusion as the mean size increases. The study incorporates bender/extender element tests capturing small-strain constrained moduli, Young’s moduli and Poisson’s ratio of RCA fractions with different mean grain size and grain size distribution. For the uniform RCA fractions, the results showed that during isotropic compression, the specimens had a clear dependency of their dynamic properties on mean grain size. For the better graded specimens, it was shown that their behavior was dominated to some extend by the finer fraction they were composed rather than being affected equally by the different fractions they consisted of. The results also highlighted the sensitivity of the behavior of the samples to the over-consolidation stress history. For RCA, a multi-composition material, the grain size has a dominant role on the modulus – pressure relationship as well as the sensitivity of material behavior to stress history and these observations must be considered in the analysis of geo-structures where RCA is used as construction material.

Tailing dam failures result in irreversible environmental impacts and cause fatalities. In recent years the mechanical behaviour of tailing geo-materials has received more attention by the geomechanics and engineering geology communities in an attempt to understand better their behaviour in the light of designing safer tailing dams. In this study, the mechanical behaviour of a gold tailing from Brazil is thoroughly investigated by conducting a series of compression and shearing tests as well as dynamic element tests. Fabric effects from the sample preparation method, the susceptibility to liquefaction and the possibility of any transitional behaviour are presented and discussed within a soil mechanics framework. Comparisons are made between the present gold tailing and previously published data on other tailings, giving a general view of the mechanics of tailings and the effects of grading. The results show that for this tailing the rate of convergence for different initial densities to the normal compression line is slow, and so the depositional density would affect the volume to far higher stresses than the material would be expected to experience in-situ. For this tailing any fabric effects from the sample preparation method were found to be very small to negligible with respect to small-strain behaviour and critical state behaviour. For different tailings, even if the particle sizes may cover a wide range, the susceptibility to static liquefaction, as determined by the location of the horizontal asymptote of the critical state line in the specific volume-log stress plane, shows no consistent variation. So it can be concluded that neither the pond nor the upper beach tailings are more susceptible.

In this paper, an experimental micromechanical study is presented investigating the contact mechanics and tribological behaviour of highly/completely decomposed tuff granules. The parent material was taken from two locations—named the top and bottom—from a recent landslide in Hong Kong, and in this study the tested granules were obtained from the parent material after drying and sieving processes. Basic material characterisation was conducted quantifying the particle shape, the surface roughness and the strength of a set of grains. A set of twenty-nine monotonic inter-particle shearing tests were conducted on pairs of granules taken from the top and bottom of the landslide. It was found that the granules had very high friction angles at their contacts, in general greater in comparison with other materials reported in the literature. The slightly greater inter-particle friction for the granules taken from the top of the landslide might be because of their higher roughness in comparison with the ones from the bottom. Additional experiments were conducted to investigate the normal and tangential load–displacement response of the granules subjected to cyclic loading. A good curve fitting for the normal load–displacement response could be obtained by using very low apparent Young’s moduli in the Hertzian model. In general, the decomposed tuff granules showed significant plastic response during the first normal load cycle, and this plastic behaviour continued for the subsequent third and fourth cycles. In the cyclic inter-particle shearing tests, the nonlinearity and hysteresis increased for larger cyclic displacements, but the effect of the number of shearing cycles on the energy loss was generally small. Finally, a limited discussion is presented on the applicability of a theoretical model on the tangential load–displacement behaviour of the granules.

Fiber-reinforced polymer (FRP) composites have a promising potential to be utilized to extend the lifespan of geostructures in harsh environments due to their high durability, satisfactory strength and cost-effectiveness. The FRP-soil interaction is critical to model soil-structure interaction problems and assess the performance of foundation systems where composites are used. In this paper, the contact behavior of sand-FRP interfaces at the fundamental level of the grain-scale was investigated with a custom-built micromechanical testing apparatus, with an emphasis on the normal contact response, the tangential load – displacement relationship as well as creep and relaxation behavior. Leighton Buzzard sand (LBS), a typical silica sand , and custom manufactured glass fiber reinforced laminated FRP plates were used in the study. It was shown that the surface texture, thickness, and fiber orientation of the FRP composite plates have important influences on the tribological behavior of the sand-FRP interfaces, and plowing damage was observed to be a dominant mechanism from the shearing tests, particularly at higher normal loads. The results also suggested that creep and relaxation had a significant influence on the interface behavior of sand-FRP and these phenomena should be considered in the long-term design of FRP included composite geostructures.

The study of the collision behavior of solid objects has received a significant amount of research in various fields such as industrial applications of powders and grains, impacts of proppants and between proppant and rocks during hydraulic fracturing, and the study of debris flows and avalanches and the interactions of landslide materials with protective barriers. This problem has predominantly been studied through the coefficient of restitution (COR), which is computed from the dropping and rebound paths of particles, its value corresponds to 1 for perfectly elastic impacts and 0 for perfectly plastic impacts (i.e., at the collision there is no rebound of the particle). Often, the colliding particles (or particle–block systems) are not perfectly clean, and there is debris (or dust) on their surfaces, forming a coating, which is a highly possible scenario in the debris flows of natural particles and fragments, however, the topic of the influence of natural coatings on the surfaces of particles on the collision behavior of particle–block systems has been largely overlooked. Thus, the present study attempts to provide preliminary results with respect to the influence of natural coating on the surfaces of sand grains in the COR values of grain–block systems using a stiff granitic block as an analogue wall. Montmorillonite powder, which belongs to the smectite clay group, was used and a sample preparation method was standardized to provide a specific amount of clay coating on the surfaces of the sand grains. The results from the study showed a significant influence of the smectite coating in the COR values of the grain–block systems, which was predominantly attributed to the dissipation of energy at the collision moment because of the compression of the soft coating of microparticles. Additionally, the method of analysis for calculating the COR values based on one and two high-speed cameras was explored, as the impacts of natural grains involve deviations from the vertical, which influences the rebound paths. Thus, a sensitivity analysis was performed investigating the differences in the COR values in two-dimensional and three-dimensional analysis of the impact tests.

The use of polypropylene fibers as a geosynthetic in infrastructures is a promising ground treatment method with applications in the enhancement of the bearing capacity of foundations, slope rehabilitation, strengthening of backfills, as well as the improvement of the seismic behavior of geo-systems. Despite the large number of studies published in the literature investigating the properties of fiber-reinforced soils, less attention has been given in the evaluation of the dynamic properties of these composites, especially in examining damping characteristics and the influence of fiber inclusion and content. In the present study, the effect of polypropylene fiber inclusion on the small-strain damping ratio of sands with different gradations and various particle shapes was investigated through resonant column (macroscopic) experiments. The macroscopic test results suggested that the damping ratio of the mixtures tended to increase with increasing fiber content. Accordingly, a new expression was proposed which considers the influence of fiber content in the estimation of the small-strain damping of polypropylene fiber-sand mixtures and it can be complementary of damping modeling from small-to-medium strains based on previously developed expressions in the regime of medium strains. Additional insights were attempted to be obtained on the energy dissipation and contribution of fibers of these composite materials by performing grain-scale tests which further supported the macroscopic experimental test results. It was also attempted to interpret, based on the grain-scale tests results, the influence of fiber inclusion in a wide spectrum of properties for fiber-reinforced sands providing some general inferences on the contribution of polypropylene fibers on the constitutive behavior of granular materials.

This study reports on the inter-particle coefficient of friction and tangential stiffness at the contacts of completely decomposed volcanic (CDV) granules taken from a landslide in Hong Kong. Micromechanical sliding experiments were conducted using a new generation inter-particle loading apparatus on pairs of granules of about 1.18–2.36 mm in size in a quasi-dry state to explore their tangential load – displacement behaviour. For this purpose, small shearing paths were applied in monotonic mode under nominal normal loads in the range of 0.5–1.0 N. The results demonstrated that the inter-particle coefficient of friction was much higher and the tangential stiffness was much lower than reported values in the literature for quartz sand grain contacts. Repeating shearing tests following the same shearing path showed a notable change of the tangential force – displacement relationship from the first to the second cycle, while subsequent cycles had a relatively small effect. Scanning electron microscope (SEM) images revealed some damage to the surface of the granules on the meso-scale, but no effect on the structure of the grains at the micro-scale was observed. It is believed that the soft nature of the grains and their high surface roughness may have contributed to these trends.

This paper reports on the small-strain stiffness (Gmax) of a well graded crushed rock reinforced with polypropylene fibres. The experimental study was conducted on twenty-five fully saturated samples subjected to bender element tests. The contents of fibre used were equal to 0%, 0.5%, 1.0%, 1.5% and 2.0%. The bender elements were inserted in a resonant column and a stress path triaxial apparatus accommodating samples of 70 and 50 mm in diameter, respectively, with a ratio of length to diameter equal to 2:1. The samples were initially consolidated isotropically to pressures of 50, 100, 400 and 700 kPa. At a given isotropic pressure, Gmax measurements were conducted increasing the deviatoric compressive stress (q) and keeping a constant mean effective confining pressure (p'). This allowed the study of the effect of stress anisotropy, expressed with the ratio (q/p'), and the role of fibre inclusion and content on Gmax of the reinforced samples under variable mean effective confining pressures. The data were analysed by means of the normalized shear modulus, Gmax,normalized, against the stress ratio, where Gmax,normalized expressed the increase of the small-strain shear modulus under an anisotropic stress state with respect to the corresponding modulus under an isotropic stress state. For a given confining pressure, the results showed a clear increase of Gmax,normalized with increased content of fibres which implied that the sensitivity of modulus to the stress anisotropy became more pronounced for greater contents of fibres. However, this influence of fibre inclusion and content on the small-strain stiffness of the samples was found weaker at greater magnitudes of p'.

Discrete element modeling requires the proper quantification of the behavior of grains at their contacts including the normal force – displacement and tangential force – displacement relationships to be used as input for contact modeling purposes. This paper reports on recent advances in soil mechanics experimentation which allowed measuring the grain contact behavior of small sand particles quantifying friction and stiffness with sliding tests of a force-controlled or displacement-controlled type. The particular focus of this work is on the micromechanical behavior of quartz type grains of size between about 1 and 5 mm. A description of the developed micromechanical apparatus at City University of Hong Kong is first discussed and its important different capabilities with previously developed apparatus is briefly reviewed. Subsequently, a limited set of new data is reported and discussed along with a review of recently acquired results published in the literature associated with the contact behavior of quartz sand grains. These sliding tests have covered a wider range of normal contact forces from about 0.5 to 8 N, and the results indicated that, for this range of confining forces, there is not any notable change of the inter-particle coefficient of friction. Keywords: Micro-mechanics, Inter-particle friction, Inter-particle stiffness, LBS grains, Experimental mechanics

The measurement of primary wave velocities (Vp) of geo-materials is an essential step for the geophysical and dynamic characterization of sediments. Advances in experimentation have established in recent years the use of the extender element test method in laboratory research and practice. This method is applicable for the measurement of Vp of geo-materials applying a high-frequency pulse excitation, typically between 5 and 20 kHz. In the study, extender elements inserted in a stress path triaxial apparatus were used for the measurement of primary-wave velocities of quartz sands of variable grain morphologies. During the dynamic tests, the samples were subjected to a deviatoric compressive load, which allowed the study of the effect of stress anisotropy, expressed through the stress ratio, on Vp. To overcome difficulties of poor signal output of P-waves, which may occur when the sample is fully saturated, the samples were prepared and tested in a dry state. Measurements of sample volumetric strains were conducted from local strain sensor attached to the sample which captured radial deformations during the isotropic and anisotropic stress paths and a vertically positioned LVDT which captured the axial strains of the samples. Accordingly, based on the measured P-wave velocities and the density of the samples, the constrained modulus (Mmax) was quantified. The results indicated that for a given mean effective confining pressure, the sensitivity of Vp and Mmax to the magnitude of the applied stress ratio was controlled, markedly, by the morphology of the grains; for sands of fairly rounded and spherical grains, this effect was relatively small, but for sands of irregular in shape grains, the stress ratio affected markedly the behavior of the samples.

This study presents a series of discrete element method (DEM) simulations consisting of mixtures of stiff (sand) and soft (rubber) particles, subjected to monotonic triaxial shearing under constant volume at very small strains where pure elasticity governs the behaviour of the samples. The elastic shear moduli of the simulated pure sand and pure rubber samples were first calibrated to values reported in previous experimental works. Sand–rubber mixtures were then simulated with a focus on small-strain stiffness to examine the role of rubber content on the prevailed micro-mechanisms of the samples. The macro-mechanical response of the numerical mixtures showed a decrease in the elastic shear modulus and the deviatoric stress as the soft particle content increased, in line with observations from laboratory tests. Micro-scale information including coordination number, fabric tensor and normal contact force anisotropy was obtained for all tests and the contribution of each type of contact, i.e. sand–sand, rubber–sand or rubber–rubber, in the overall response of the samples, was analysed. The contact force network in the mixtures changed from being sand-dominated to rubber-dominated, with the presence of an intermediate zone in between rubber and sand particles forming a stable contact force network mainly by sand–rubber contacts. Each type of contact was seen to contribute differently to the deviatoric stress in the system as the rubber content increased.

Particle crushing has been recognised as being of key importance for many engineering applications. In soil mechanics, this phenomenon has become crucial for defining a complete framework capable of describing the mechanical behaviour of sands. In this study, the effect of multiple discrete contacts on the breakage of grain particles was investigated by crushing coarse grains of a quartz sand and a crushed limestone sand between a number of support particles, thereby varying the number of contacts, i.e., the coordination number. The stress at failure was calculated when the particles broke, which occurred through a number of distinct modes, namely, chipping, splitting or fragmenting, which were observed with the use of a high-speed microscopic camera. The Weibull criterion was applied to calculate the probability of surviving grain crushing, and the fracture modes were observed for each configuration of supporting particles. The data showed that in addition to the number of contacts, the nature of those contacts, controlled by particle morphology and mineralogy, also plays a significant role in determining the strength of a particle. The sphericity affected the strength of the softer limestone sand, while the local roundness at the contacts was important for the harder quartz sand. Catastrophic explosive failure was more often observed in particles with harder contacts, while softer contacts tended to mould relative to their neighbouring particles inducing a more frequent ductile mode of crushing.

Sedimentary deposits often display a variety of grains and especially in gap graded materials or soil mixtures, it is expected that the finer particles may develop a coating in the contact region of coarser particles. Development of coatings in the vicinity of the contacts of coarser grains may also be encountered due to increase in depth and pressure which cause breakage of grain asperities forming in this way smaller particles which interact with larger grains and coatings may also influence reservoir quality. As the interactions of grains at their contacts have a significant influence on the mechanical, geophysical, hydraulic and thermal properties of sediments, it is essential to be examined in the laboratory through grain-scale tests to provide insights into their behaviour. In the present study, a new technique was developed in coating sand grains with siliciclastic non-plastic powder and emphasis was placed in the assessment of the mechanical behaviour of the specimens, comparing them with plastic-coated grains. The experiments were performed with grain-scale methods and were further supported with direct shear tests. The results showed that different frictional mechanisms involve in non-plastic coated grain contacts compared with the case of plastic coatings and that the influence of loading history played an important role on the tribological behaviour of the grains. Microcrushing and rearrangement of the siliciclastic non-plastic coating were the major contributing mechanisms, and their influence was found to be dependent on the magnitude of the applied normal load, which was not the case of plastic-coated grains. The results from the study provide fundamental understanding on the role of the type of natural coatings on the interface behaviour of sediments and highlight the important differences at the grain-scale between sand-silt and sand-clay mixtures.

Mixtures of sand and rubber particles were simulated using DEM. Rubber content varied from 0% to 50%. The numerical samples were sheared in the range of 10 −5 –10 −2 % of e 1 . The macro-mechanical response changed depending on the size of rubber particles. As the size of the rubber particles increased, the effect of rubber in the internal structure was attenuated, facilitating the force transmission through sand-sand contacts. Largest rubber particles showed the most advantageous mechanical behaviour. Nevertheless, the selection of both rubber size and content will depend on the intended purpose of use for the mixtures.